This invention concerns a method for determining a humidity content of an insulation of a transformer, and a device of corresponding form.
Humidity, in particular in the case of oil-paper insulations of power transformers, reduces their electrical strength, makes them age faster and can result in formation of gas bubbles in the insulation. The humidity content of the insulation is understood to mean a percentage water content in the insulation. Knowledge of the humidity content in solid parts of the insulation, e.g. paper or pressboard, is of great importance for safe operation of power transformers. For measuring the humidity content in the solid parts of the insulation, dielectric measurement methods, which derive the humidity content from one or more dielectric properties of the insulation, have been developed. This measurement can be done in the time or frequency domain, the results being measured magnitudes or dielectric properties such as polarization and depolarization currents, the dissipation factor tangens delta, the dielectric constant. From these measured magnitudes, by comparison with a model, the humidity content in the solid part of the insulation or the solid insulation can be calculated.
It is problematical that the dielectric properties of the insulation can be affected not only by water, but also by other conductive aging products, so that additional humidity can be simulated by these aging products. Therefore, the measured or analyzed humidity content or water content is typically too high compared with the actual humidity content, in particular in the case of very aged transformers with a relatively highly conductive insulation system. This wrong diagnosis because the humidity content is wrongly too highly measured can result in unnecessary maintenance actions, e.g. drying.